1. Field of the Invention
This disclosure relates to carbon nanotubes having improved electrical conductivity, a process of preparing the same, and an electrode comprising the carbon nanotube. This disclosure also relates to a method of producing carbon nanotubes having improved electrical conductivity and to a method of manufacturing an electrode that comprises the carbon nanotubes.
2. Description of the Related Art
In general, devices such as display devices and solar cells that transmit light to form images or to generate electric power use transparent electrodes. Indium tin oxide (ITO) is widely known as a transparent electrode and has a wide range of applications. However, the increased costs of manufacturing ITO, render it uneconomical in many applications. In particular, when a transparent electrode formed of ITO is bent, it begins to undergo cracking thereby causing an increase in its electrical resistance. Accordingly, the use of ITO electrodes may cause deterioration of quality in flexible devices, and there is therefore a need to develop a novel electrode that is optically transparent and that can be used in flexible devices. A transparent electrode including carbon nanotubes can be used in a wide range of devices, such as liquid crystal display (LCD) devices, organic light emitting display devices (OLEDs), electronic paper like displays, or solar cells.
A transparent electrode including carbon nanotubes must have conductivity, transparency, and flexibility. Generally, a transparent electrode including carbon nanotubes are prepared by dispersing a carbon nanotube powder in a solution to prepare a carbon nanotube ink and then coating the carbon nanotube ink on a substrate. The prepared transparent electrode including the carbon nanotubes has a network structure formed of carbon nanotubes. As a result, electrons flow in the carbon nanotubes themselves and between the carbon nanotubes to function as an electrode. Accordingly, conductivity of the electrode including the carbon nanotubes is determined by flowability of the electrons in the carbon nanotubes themselves and between the carbon nanotubes.
According to the results of recent research, in the electrode having the lattice structure of carbon nanotubes, when the number of carbon nanotubes is sufficiently large that the carbon nanotubes can contact each other, that is, when the number of carbon nanotubes is equal to or higher than a critical number, a carbon nanotube network film is not affected by the resistance of the carbon nanotubes themselves and mainly affected by the contact resistance between the carbon nanotubes (Nanoletter 2003, 3, 549). Thus, reduction in the contact resistance between the carbon nanotubes is critical to improvement of the conductivity of transparent electrode including carbon nanotubes. According to the results of other recent research, it was found that contact conductivity varies because of properties of a mixture of semiconducting and metallic carbon nanotubes (Science, 288, 494). The sheet resistance of the random network SWCNTs is determined by the sum of resistances of the intrinsic SWCNT network and tube-tube contact. The tube-tube contact is composed of metal-metal and semiconductor-semiconductor junctions that give ohmic behavior, and a metal-semiconductor junction that forms a Schottky barrier. When electrons flow from a semiconducting carbon nanotube to a metallic carbon nanotube, a Schottky barrier is generated causing relatively low contact conductivity. Thus, there is a need to increase contact conductivity of sCNT-sCNT or sCNT-mCNT, or to reduce the contact amount.